Voltage regulators for use in battery charging systems

ABSTRACT

A voltage regulator is disclosed for varying the output of a generator of a road vehicle by changing the current flow in its field winding. The voltage regulator device includes, as is common, a Zener diode or equivalent break-down device which, as known, conducts at a predetermined battery voltage, and serves to render a transistor conductive, which transistor, when rendered conductive, acts through further components to reduce the current flow in the field winding. In systems of this type it is commonplace to provide for temperature compensation by providing a thermistor or equivalent device in the circuit. Such device, in a known manner, serves to compensate for changes in battery temperature, as well as for the changes in temperature coefficient of the Zener diode and the transistor so utilized. However, in accordance with the instant invention, such temperature compensation is achieved by means of a simple impedance network interconnecting the Zener diode or equivalent device and the transistor, such impedance network, in effect, serving to multiply the net negative inherent temperature characteristics of the transistor by a selected amount to compensate for the net positive temperature coefficient of the Zener diode, the resultant characteristics of the voltage regulator circuit with respect to temperature being desirably negative or, at least, zero. In the specific preferred embodiment, a resistor is disposed across the base-emitter of the transistor, and another resistor is disposed in series circuit with the Zener diode. The ratio of these two resistors substantially defines a multiplication factor which enhances the net negative temperature coefficient of the transistor and serves to overcome the net positive temperature coefficient of the Zener diode.

United States Patent [1 1 Nolan et al.

[451 Feb. 27, 1973 VOLTAGE REGULATORS FOR USE IN BATTERY CHARGINGSYSTEMS [75] Inventors: Roger William Nolan, Redditch; Maurice JamesWright, Quinton, both of England [73] Assignee: Joseph Lucas(Industries) Limited,

Birmingham, England [22] Filed: June 3, 1971 [21] Appl. No.: 149,585

Related US. Application Data [63] Continuation-in-part of Ser. No,93,876, Nov. 30, 1970, abandoned, which is a continuation of Ser. No.701,892, Jan. 31, 1968, abandoned.

[30] Foreign Application Priority Data Feb. 6, 1967 Great Britain..5,566/67 [52] US. Cl. ..320/61, 322/28, 323/38 [51] Int. Cl ..H02j7/10 [58] Field of Search ..322;28/; 323/38; 320/61, 38, 320/68, 39, 32

[56] References Cited UNITED STATES PATENTS 3,373,333 3/1968 Eckard..320/32 3,538,421 11/1970 Young ..323/38 3,201,681 8/1965 Van Wilgrenet al ..320/68 X Primary Examiner.l. D. Miller Assistant Examiner-RobertJ. Hickey Attorney-Holman & Stern [57] ABSTRACT A voltage regulator isdisclosed for varying the output of a generator of a road vehicle bychanging the current flow in its field winding. The voltage regulatordevice includes, as is common, a Zener diode or equivalent break-downdevice which, as known, conducts at a predetermined battery voltage, andserves to render a transistor conductive, which transistor, whenrendered conductive, acts through further components to reduce thecurrent flow in the field winding.

in systems of this' type it is commonplace to provide for temperaturecompensation by providing a thermistor or equivalent device in thecircuit. Such device, in a known manner, serves to compensate forchanges in battery temperature, as well as for the changes intemperature coefficient of the Zener diode and the transistor soutilized. However, in accordance with the instant invention, suchtemperature compensation is achieved by means of a simple impedancenetwork interconnecting the Zener diode or equivalent device and thetransistor, such impedance network, in effect, serving to multiply thenet negative inherent temperature characteristics of the transistor by aselected amount to compensate for the net posithe temperaturecoefficient of the Zener diode, the resultant characteristics of thevoltage regulator cirquit with respect to temperaturebeing desirably neative or, at least, zero. In he specific preferred embo ment, a resistoris disposed across the base-emitter of the transistor, and anotherresistor is disposed in series circuit with the Zener diode. The ratioof these two resistors substantially defines a multiplication factorwhich enhances the net negative temperature coefficient of thetransistor and serves to overcome the net positive temperaturecoefficient of the Zener diode.

4 Claims, 6 Drawing Figures VOLTAGE REGULATORS FOR USE IN BATTERYCHARGING SYSTEMS This application is a continuation-in-part applicationof co-pending Application Ser. No. 93,876, filed Nov. 30, 1970 nowabandoned which in turn is a continuation of Application Ser. No.701,892, filed Jan. 31,

1968 now abandoned.

The invention disclosed herein relates to voltage regulators for use inbattery charging systems on road vehicles, and particularly concerns anovel voltage regulator circuit which achieves temperature compensationin a unique manner.

Voltage regulator circuits and devices for use in vehicular batterycharging systems have long been known in the art and are generally seento comprise a pair of terminals adapted for connection to the vehiclebattery with a series circuit coupled across the terminal pair andincluding a voltage sensitive device such as a Zener diode and aresistor, the voltage sensing device being adapted to conduct at somepredetermined battery voltage so as to develop an output signal acrossthe resistor. A transistor device is normally also provided with thedevice having its base-emitter junction connected across the resistor.The collector output circuit of the transistor is normally coupled tothe field winding of a generator, for example, such that the collectorcurrent of the transistor effectively varies the output of the generatorwhich charges the battery. ln known manner, the overall regulatorcircuit is such that regulation will commence at some predeterminedtransistor collector current.

Problems arise, however, with circuits of this type, such problems beingspecifically related to temperature compensation. As is known, inpractice, the temperature of the vehicular battery inevitably varies andit thus becomes necessary to provide some type of temperaturecompensation. In addition, as the temperature of the ambient environmentvaries, the particular characteristics of the individual components,such as the Zener diode and the transistor, likewise v'ary, thisvariation in and of itself being deleterious to proper operation of theregulator circuit. For example, it is known that the normal type ofZener diode used in such a regulator has what can be termed a netpositive temperature coefficient such that, as the temperatureincreases, the breakdown for conduction voltage of the Zener diodelikewise increases. Even though the transistor devices utilized in suchcircuits have inherently negative temperature coefficients in that, fora given output collector current, the input base voltage neededdecreases at increasing temperatures, the net positive temperaturecoefiicient of the Zener diode will be seen to effectively swamp oroverride the negative transistor characteristics such that the overalltemperature coefficient of the regulator circuit is positive. Again,such positive characteristics are inappropriate to proper regulationtechniques as will be apparent from a consideration of a paper entitledThe Battery, The Voltage Regulator and the need for System Concept",delivered by R.W. MacKay, of Prestolite Corporation, at theInternational Automobile Engineering Congress held at Detroit on January1 1-15, 1963.

In an effort to overcome these disadvantages of the typical and basicregulation circuit above-described the prior art has gone to theutilization of a thermistor chosen device, for example, coupled in theZener diodetransistor circuit, the thermistor device functioning inknown manner to compensate for the otherwise deleterious temperaturecoefficient of the regulator circuit. Yet, even the provision of such athermistor device has its disadvantages in that such devices are, forone, expensive and, further, cannot easily be manufactured withmicro-circuitry techniques and the like. Additionally, it is difficult,when one considers the manufacture of a large number of regulatorcircuits, to find a correspondingly large number of thermistors havingsuitable tolerances.

As one of its primary objectives, the instant invention contemplates theprovision of a regulator circuit of the above basic type, but whereinthe thermistor so prevalent in prior art constructions is eliminatedwhile an overall advantageous temperature coefficient of the circuit isretained. In this respect, the novel invention contemplates theprovision of an impedance network, such as resistors, in circuit withthe Zener diode or other breakdown device and the transistor, thisimpedance circuit being such that the net inherently negativetemperature coefficient of the transistor is effectively multiplied by apredetermined amount sufficient to overcome the net positive temperaturecoefficient of the Zener diode as above discussed.

Specifically, the instant invention contemplates the provision of aregulator device which, in standard fashion, includes a pair ofterminals for connection to the battery. A series. circuit is likewisecoupled across the pair'of terminals and includes a voltage sensitivedevice such as a Zener diode and resistors, one such re sistor beingcoupled across the base-emitter junction of a transistorlikewise'coupled across the pair of terminals. A further resistorlikewise is coupled in series circuit arrangement with the breakdowndevice.

In the usual fashion, means are provided which are sensitive to thecollector current of the transistor for varying the output of thegenerator which charges the battery. However, temperature compensationis achieved in a unique manner in that the value of the resistor coupledacross the base-emitter junction is so in relation to thecharacteristics of the transistor and the other circuit value such thatthe battery voltage at which regulation commences varies withtemperature in a desired, predetermined manner. In the preferredinventive embodiment, the ratio of the resistor disposed in seriescircuit with the Zener diode and the resistor coupled across thebase-emitter circuit of the transistor substantially defines amultiplication factor, which factor serves to inherently multiply thenet negative temperature coefficient of the transistor by an amountsufficient to overcome the net positive temperature coefficient of theZener diode so as to render an overall regulator circuit temperaturecoefficient that is negative or is at least zero as is desirable in thisart.

As a more detailed, though still exemplary embodiment of the instantinvention, let it be assumed that the transistor selected has a knowngiven negative temperature coefficient of -t,. Let it further be assumedthat the Zener diode so utilized has a positive net temperaturecoefficient of +t,,. As further definition, let the resistor coupled toand disposed in series circuit with the Zener diode be termed R and letthe resistor coupled across the base-emitter circuit of the transistorbe termed R Now, and as will be shown herein below, suitable selectionof these resistor values can serve to effect a resultant net negative orat least zero temperature coefficient of the overall circuit.Specifically, the ratio of resistors or impedances in the manner of R lRor, more precisely, (R +R )/R substantially defines a multiplicationfactor F. In the circuit as described, this multiplication factor F willbe seen to multiply the negative net temperature coefficient of thetransistor in the manner (F) (t,,) so that=this value equals or exceedsthe absolute value of the net positive temperature coefficient of theZener diode, (ty), whereby an overall negative or a substantially zerotemperature coefficient is achieved.

As should be apparent, merely through the selection of suitableimpedance values as set forth above, temperature compensation in aregulator circuit of the type above-described is achieved and, as shouldbe appreciated, this in and of itself represents a markedadvance in theart. By virtue of this technique, those of ordinary skill in the art canapply these teachings to other similar circuitry and, by merelyutilizing impedance values in accordance with the above concept, selectthe overall temperature coefficient of the circuit.

The invention itself will be better understood and other advantages andobjectives thereof will become apparent when reference is given to thefollowing detailed description thereof, such description makingreference to the appended drawings, wherein:

FIG. 1 is a circuit diagram illustrating one example of the instantinvention as applied to a battery charging system which employs analternator;

FIG. 2 is a fragmentary view of a portion of FIG. 1 illustrating amodification of the instant invention;-

FIG. 3 is a graph of regulating voltage against ambient temperature forvarious regulator types useful in explaining the overall result of theinstant invention; and

FIGS. 4, 5 and 6 are schematic diagrams of basic regulator circuitsuseful in teaching the principles of the instant invention.

As background of the instant invention and so as to better understandand appreciate the principles thereof, attention is initially directedto FIG. 4 of the drawings. In FIG. 4, the basic regulation circuit isdepicted and will be seen to comprise a terminal pair 60 and 62 acrosswhich terminal pair is connected a transistor 64 in so-called groundedemitter configuration. A voltage break-down element such as Zener diode66 is coupled to the base circuit of the transistor 64. Such a circuitis, as stated, typical of regulation circuits and, per se, is well knownin the art.

Now, considering the basic operation of this circuit, if the voltage ofV is below a desired or predetermined value, depending upon thetransistor characteristics, then no current would flow into thetransistor base, and therefore no collector current would flow from thetransistor. As referred to above, such collector current is normallyutilized in such circuits to control the alternator or generator of theregulation system. Thus, with this condition, the alternator wouldreceive maximum field amperage since no regulation would be present.

If the circuit conditions were such that the system voltage V risesabove some desired level, then the voltage V would exceed the givenbreak-down voltage of the Zener diode 66 and the Zener diode wouldconduct providing current to the emitter base circuit base of thetransistor, and thus output collector current from the transistorutilized for regulation control. Accordingly, the amount of currentflowing in the field winding of the system alternator or generator woulddecrease reducing the system output so as to maintain the system outputat its desired value.

However, due to some practical as well as theoretical considerations,the basic regulation network abovedescribed is seldom left in thissimple form. For one, such a simple circuit requires the provision of aZener diode of very close tolerances and quite precise values. Suchdiodes are difficult to obtain in quantity. Furthermore, theabove-discussed basic circuit suffers a disadvantage with respect to itsoverall temperature coefficient and characteristic. 7

Specifically, the temperature coefficient of the circuit voltage isdirectly dependent upon the break-down voltage of the Zener diode 66.For a nominal 5-volt Zener diode, the temperature coefficientapproximates zero as is known in the art. However, this temperaturecoefficient increases in a positive manner for increasing voltages above5-volts across the Zener diodes. As a typical example, and assuming thatthe Zener diode 66 is a nominal l3-vo1t diode manufactured by TexasInstruments lS2l30A, such a diode will be seen to have a positivetemperature coefficient of +0.07 percent per degree Centigrade. Such'apositive temperature coefficient is unfortunately high for batterycharging circuits since it results in a net rise of system voltage withincreasing temperature. As is known, such a positive temperaturecoefficient circuit is highly undesirable in this environment. I

To prove this net positive temperature coefficient with moremathematical particularity let us look at what occurs at a 25 C. ambienttemperature. The voltage V across the Zener diode 66 will nominally be13.00 volts whereas the voltage V, across the base emitter path oftransistor 64 would nominally be 0.70 volts which is typical of silicontransistors of the specific type enumerated above, for example. Thus,the overall system voltage V, at 25 C. would be approximately 13.7volts.

At C. ambient temperature, different results follow due to thetemperature coefficient of the Zener diode 66 and the transistor 64. Forexample, and assuming the element types above discussed, at thistemperature the voltage across the Zener diode V would rise by thepositive temperature coefficient multiplied by the temperature rise,that is by 0.07 percent of 13x50, which equals 0.455 volts, giving atotal of 13.455 volts across the Zener diode V As is also known, for asilicon or germanium transistor, the temperature coefficient of 'thebase-emitter voltage V is approximately minus 2.5 millivolts per degreeCentigrade. Thus, at the 50 C. increase in temperature contemplatedhere, the voltage V would be reduced by 0.125 volts to form a new valueof 0.575 volts. Adding together the voltages across the Zener diode andacross the base-emitter path'of the transistor, a new system V would befound to approximate 14.03 volts. As can be seen, an increase in ambienttemperature has resulted in a net positive increase in system voltageand thus a net positive temperature coefficient of the circuit exists.

With the above now firmly in mind, the further development of the art inthis area will be discussed and here attention is specifically directedat FIG. 5. Generally speaking, Zener diodes render rather poor operationdue to their leakage currents at low current values. In such circuits,it is usual to have at least 1 to 5 milliamps in the Zener diode duringregulation and, in the circuit described in FIG. 4 as well as now shownin FIG. 5, in order to have a final control current of zero to 1milliamp through the transistor collector, the current flowing throughthe Zener diode 66 would be any where in the range from zero to 50microamps. This calculation assumes a nominal and typical transistorgain of 20. In order to enable the Zener diode to conduct this currentbefore actual base current slows into the transistor 64, a resistor R,is provided across the base-emitter path of the transistor. Suchresistor is necessary to complete acircuit path through the Zener diode66 at the time when the transistor 64 still is not conducting since itsbase-emitter voltage V, is not suffi ciently high.

Furthermore, and referring here specifically to FIG. 6, the Zener diode66 is easily damaged by ripple voltages as well as voltage excursions ofthe system voltage V, due to the high currents which can be drawnthrough the Zener diode 66 and the transistor 64 which has a relativelylow slope resistance. Accordingly, it is therefore desirable to providesome resistance in series with the Zener diode, such as resistor R, soas to limit the current and power surges to safe values which can easilybe handled by the semi-conductor components. Thus, though normaldevelopment, regulatory circuits of the prior art are thus generally ofthe type shown in FIG. 6.

Yet, the circuit shown in FIG. 6, and assuming normal or prior artvalues of resistances R, and R,, still suffers from temperaturecompensation problems in that the net or overall temperature coefficientof the circuit undesirably remains positive much as was indicated withrespect to the circuit of FIG. 4.

To show this undesirable result, typical prior art values can besubstituted for the resistors R, and R, such as ohms, and 560 ohms,respectively. The remaining components will be assumed to be standard asdiscussed above. With these substituted values, the following resultscan be seen with respect to temperature compensation.

The system will commence regulation when the transistor 64 is broughtinto conduction to an extent where it causes a reduction of fieldcurrent. That is, V increases until R, is passing sufficient current todevelop 0.7 volts across it, since transistor 64 cannot conduct untilthis situation exists. Thus, the current in R, is given by n, 0.7/5601.2mA

However, the 0.7 volt is only true at one particular temperature and, infact, the 0.7 volt decreases by 2.5 millivolts per degree Centigrade.Since it is the voltage of the base-emitter which determines the currentin R, we have more strictly (0.7V 2.5mV/C)/560 =1.20mA-4.46 microampslCThis current passes also through the Zener diode and the resistor R,. Inthis analysis we shall ignore the current required between base andemitter of the input transistor. This is legitimate since in a practicalcircuit it can be kept small by suitable design, and this is done anywayin order to predict accurately what the temperature coefficient of acircuit will be.

The voltage of regulation is then given by the sum of the voltagesacross R,, the Zener diode, and R,. With a typical 13 volt Zener diodein a 12 volt system, the Zener diode having a 0.07 percent per degreesCentigrade temperature coefficient, this gives +9.1 millivolts perdegrees Centigrade for the 13 volt diode.

V 13 V 9.1 millivolts /C It is assumed here that the current of 1.2milliamps is sufficient to bring the Zener into the avalanche voltageregion which would be the case in any sensible design.

Lastly, the voltage drop across R is given by its value multiplied bythe current flowing through it. This current is the same as 1 B, 10(l.2mA-4.46microamps/C) l2mV- 44.6microvolts/C Adding the three voltagestogether, the voltage V, at regulation is given by V, 0.7 volts 2.5millivolts per degree Centigrade.

+13.0 volts 9.1 millivolts per degree Centigrade. 12 millivolts 44.6microvolts per degree Centigrade. I 13.712 volts (6.6 0.0446) millivoltsper degree Centigrade. It will be seen that the voltages contributed bythe values of R, and R, are completely negligible since they give avoltage drop of only 12 millivolts with a -44.6 microvolt per degreeCentigrade variation to add to the Zener and base emitter. In otherwords, the circuit quite clearly possesses an overall undesirable netpositive temperature coefficient.

Yet, such a net positive temperature coefficient does not necessarilyhave to result with a circuit similar to that shown in FIG. 6 providedthat a proper selection of resistors R, and R, is made. In this respect,applicants have discovered that if these resistors are properly chosen,a net negative overall temperature coefficient or at least a zerotemperature coefficient can result. It is this discovery which lies atthe very heart of applicants invention. In the foregoing calculations,it has been seen that the positive temperature coefficient of the Zenerdiode efiectively swamps the inherently negative temperature coefficientof the transistor rendering the resultant or overall temperaturecoefficient of the regulatory circuit undesirably positive. By properselection of the value of resistor R, and R,, this inherently negativetemperature coefficient of the transistor can effectively be multipliedto overcome the positive temperature coefficient of the Zener diode.

From a conceptual point of view, the novel invention will be seen tohave the following general characteristics as concerns the selection ofresistance values. In a circuit of this general type such as discussedin FIG. 6, the positive temperature coefficient of the Zener diode 66 isknown upon selection of the particular element. Likewise, the inherentnegative temperature coefficient of the transistor is known once aparticular transistor type has been selected. Applicants have found thata ratio of the resistors R IR or, more precisely, (R, +R )/R in fact,substantially defines a multiplication factor F which, in circuit, willmultiply the negative temperature coefficient of the transistor 64,which coefficient will be termed -t,. Again from a conceptual point ofview, one need merely select values of resistance of the resistors R andR such that the multiplication factor is high enough so that thismultiplication factor acting upon the negative temperature coefficientof t, of the transistor 64 overcomes, i.e., equals or exceeds, theabsolute value of the positive temperature coefficient of the Zenerdiode 66, which temperature coefficient for purposes of illustrationwill be termed +t,,.

Applicants have therefore discovered that one may select any value ofresistances R and R to satisfy the following equation:

whereby a net negative or at least zero overall circuit temperaturecoefficient will result.

At this point, attention is directed to FIG. 1 of the drawings whereinthe actual preferred and exemplary circuit of the instant invention isdisclosed. Following thisdescription, it will be seen that the valuesselected for the resistors corresponding to resistors of R and R ortheir equivalents, in FIG. 6, fall within the above equation, and thatthis circuit, merely through suitable selection of resistance values inaccordance with the instant invention, exhibits a netnegative-temperature coefficient.

Referring to FIG. 1 an alternator 11 supplies power to a full waverectifier 12 to positive and negative supply lines 13, 14 between whichthe battery 15 of a road vehicle is connected. The alternator alsosupplies power through three additional diodes 16 to a positive supplyline 17, which in use will be at substantially the same potential as thepositive line 13. The lines 17, 13 are interconnected by a warning lamp18 in series with the ignition switch 19 of the vehicle, a resistor 20being connected across the lamp 18.

Connected across the lines 13, 14 are a pair of resistors 21, 22 inseries, the values of these resistors being such that the current drainwhen the vehicle is not in use is negligible. A point intermediate theresistors 21, 22 is connected to the cathode of a Zener diode 23, theanode of which is connected, to the line 14 through a resistor 24, andis further connected to the base of an n-p-n transistor 25, the emitterof which is connected to the line 14. The collector of the transistor 25is connected to the line 17 through a resistor 26, and is furtherconnected to the base of an n-pn transistor 27, the collector of whichis connected to the line 17 through a resistor 28, and the emitter ofwhich is connected to the base of an n-p-n transistor 29. The transistor29 has its emitter connected to the line 14, and its collector connectedto the line 17 through the field winding 31 of the alternator, a diode32 being connected in parallel with the winding 31. The collector of thetransistor 29 is also connected to the base of the transistor 25 througha feedback path including a resistor 33 and a capacitor 34 in series.

In operation, when the ignition switch 19 is closed,- the transistors27, 29 are turned on by current flow through the warning lamp 18, whichis illuminated. Field current now flows in the winding 31. As soon asthe alternator 11 produces an output, the potential of the line 17 risesto that of the line 13, and so the warning lamp 18 is extinguished,although the transistors 27 29 are still maintained conductive by powersupplied through the diodes 16. Maximum field current now flows.

When a predetermined voltage is obtained, the Zener diode 23 conductsand a voltage is developed across the resistor 24. The resultant baseemitter current in the transistor 25 causes collector current to flow inthe transistor 25, and when this collector current reaches apredetermined value, sufficient current flowing through the resistor 26is diverted through the transistor 25 to cause a switching action tocommence. The switching action causes the transistor 25 to become fullyconductive and the transistors 27, 29 to be turned off. The fieldcurrent circulates through rectifier 32 and commences to decay. Thefeedback path through the resistor 33 and capacitor 34 ensures that thecircuit switches rapidly from one state with the transistor 25 on andthe transistors 27, 29 off, and a second state in which the transistors27, 29 are on and the transistor 25 is off. The mark-space ratio isdetermined by the current flowing through the Zener diode 23, which inturn is dependent upon the voltage of the battery, and the arrangementis such that the mean current flow in the winding 31 maintains thebattery voltage at a predetermined value.

As mentioned at the outset, if the battery temperature was constant, thecircuit would be quite satisfactory as described above. However, inpractice the temperature of the battery inevitably varies, and so it isnecessary to provide temperature compensation. As remarked, suchcompensation is normally accomplished by utilizing a thermistorconnected in parallel with the resistor 21, and in FIG. 3 the curvemarked A shows the relationship between temperature and regulatingvoltage for a known regulator using a thermistor. It will be seen thatthe characteristic which is preferable is a falling regulating voltageas the temperature increases. If the thermistor is omitted, curve B isobtained, and this curve is totally unsatisfactory. The curve C isobtained without a thermistor provided the values of the variouscomponents are correctly chosen in accordance with the invention, asdiscussed. The transistor 25 has an inherent characteristic such thatfor a predetermined collector current, the base-emitter voltage requireddecreases with temperature. Since regulation commences at apredetermined collector current, it can be arranged that the outputvoltage of the alternator is reduced as the temperature of thetransistor 25 increases, and so regulation can be obtained provided thatthe transistor 25 experiences temperatures which are sufficientlyclosely related to the battery temperature. It must be noted that inorder for this compensation to be effected, the value of the resistor 24must be carefully chosen in relation to the value of the resistor 21,because the effective compensation is multiplied by the ratio of theseresistors. Circuits are known of the general form shown in FIG. 1 as perFIGS. 4 through 6 in which no temperature compensation is providedwhatsoever. Such circuits .are not suitable for use in a batterycharging system, because of the problems mentioned above, and a circuitshown in FIG. 1 distinguishes from such circuits particularly in the wayin which the relationship between the values of the resistors 21 and 24is chosen. A typical, though exemplary set of values for FIG. 1 is givenbelow:

component component type 21 l,000 ohms 22 3,000 ohms 25 Lucas DT I6 24200 ohms 27 Lucas DT 16 26 1,000 ohms 29 Lucas DT 32 28 150 ohms 23 8VZener diode 33 1,000 ohms 34 10,000 picofarads The actual nature of thevariation of regulated voltage with temperature depends on theparticular application, and so can take a variety of forms. Since thecharacteristics of the transistor 25 previously referred to is linear,the compensation will also be of a linear nature, but if desired thecompensating law can be changed one or more times at varioustemperatures. FIG. 2 illustrates a modification which enables this to bedone. The circuit is the same as FIG. 1 except that there is added aresistor 41 connected in series with a resistor 42 across the lines 13,14 the junction of the resistors 41, 42 being connected through a secondZener diode 43 to the base of the transistor 25. It can easily bearranged for this circuit to ensure that the output voltage of thealternator is constant until a temperature of 20C is reached, afterwhich the output voltage of the alternator falls with further increaseof temperature. In

order to do this, the ratio of the resistance of resistors 21, 24 ischosen to give the constant voltage up to 20C, and the ratio of theresistors 41,24 is chosen to give the required slope above 20C. In orderto set such a circuit, the resistors 22, 42 are first made with too higha value. The value of the resistor 22 is first set for operation belowthe temperature of 20C, whereafter the value of the resistor 42 is set,with the circuit at an elevated temperature, to produce the requiredcharacteristics.

As mentioned, the circuit shown in FIG. 1, due to the selection of thevalues of resistances R, and R, in accordance with the general teachingsof the instant invention as above-discussed, does, in fact, exhibit anoverall negative temperature coefficient, and, in this manner, markedlydiffers form circuits of similar type in the prior art.

The operation of the impedance elements 21 and 24 to magnify thenegative temperature coefficient of the transistor may be analyzed interms of the voltages across the impedance elements to achieve a changein conduction of the transistor. For a silicon transistor, the baseemitter voltage which is necessary to bring about a change of conductionis on the order of 0.7 volts. This voltage decreases with increasingtemperature by 2 l: millivolts per degree Centigrade. If resistor 24 hasa typical value of 200 ohms, the current passed through this resistor bythe circuit in order to bring about this change of transistor conductionis 3 95 milliamps. This current decreases with increasing temperature byan amount proportional to the temperature coefficient of the transistorWith the given parameters this current decrease comes to 12 microampsper degree Centigrade.

All of the current which flows through resistor 24 also flows throughresistor 21. Slightly more than 3 milliamps minus 12 microamps perdegree Centigrade must flow through resistor 21 in order to bring abouta change in conduction of the transistor. Of course, some of the currentwhich flows through resistor 21 also flows through the other dividerresistor 22. The current through resistor 22 can be estimated at 2 Vamilliamps, with an 8 volt Zener in the circuit. Also, some of thecurrent through the resistor 21 passes through the base emitter junctionof the transistor. However, for the purposes of this conceptualanalysis, we can neglect the base to emitter current since its value iskept small compared with the current in resistor 24.

We are now in a position to estimate the potential difference developedacross resistor 21, which resistor has an exemplary value of 1,000 ohms,by a current which is sufficient to bring about a change in conductionof the transistor. This current will be 2 k milliamps plus 3 iimilliamps, or 6 milliamps, and the effect of the temperature coefficientof the transistor is to reduce this by 12 microamps per degreeCentigrade resulting in a voltage change of 1,000 X 12 or 12 millivoltsper degree Centigrade. Thus, the voltage across resistor 21 at the pointwhere the transistor changes conduction is 6 volts less 12 millivoltsper degree Centigrade.

The voltage across the other divider resistor is the Zener referencepotential plus the control voltage "of the transistor. Given the presentstate of the art, Zener diodes have certain characteristics in common.It is known that above about 5 volts, Zeners have an increasing positivetemperature coefficient. A 13 volt Zener INZA made by Texas InstrumentsInc. has a temperature coefficient of about 0.07 percent per degreeCentigrade. An 8 volt Zener will have less of a coefficient than that,but taking the 13 volt value as a worse case, the Zener referencepotential will increase 5.6 millivolts per degree Centigrade. As we havesaid, the transistor control potential is about 0.7 volts minus 2 hmillivolts per degree Centigrade. Therefore, the voltage across resistor22 is 8.7 volts and increases with temperature at 5.6 minus 2.5 equals3.1 millivolts per degree Centigrade.

It will be noted that the net increase of potential across resistance 22with increased temperature is just what the prior art shows. Without themultiplying effect of resistors 21 and 24 the positive temperaturecoefficient of the Zener swamps the negative temperature coefficient ofthe transistor and the temperature coefficient of the circuit ispositive.

Now, however, we are in a position to see the combined effects ofresistors 21 and 24 and the rest of the circuit. The voltage across thevoltage divider made up of resistors 21 and 22 when the conduction ofthe transistor changes will be the VR VR VR, 6 volts 12 millivolts perdegree Centigrade VR 8.7 volts 3.1 millivolts per degree Centigrade.

Clearly, the total circuit has a negative temperature coefficient ofabout minus 9 millivolts per degree.

I degree Centigrade.

Dividing by 100 gives the current flowing in R i.e.,

I 7 milliamps 25 microamps per degree Centigrade This current flows in Ralso developing a voltage across R which varies with temperature R alsopasses the current required by R With a 10 volt Zener in circuit, thislatter current is given by (10 0.7)/3,500 3 .03 milliamps Total currentin R 10.03 milliamps 25 microamps per degree Centigrade Voltage across R3.01 7.5 millivolts per degree Centigrade. The voltage of regulation isgiven by the sum of the three constituent voltages.

V 0.7 volts 2.5 millivolts per degree Centigrade.

V, 10 volts 7.0 millivolts per degree Centigrade.

V 3.01 volts 7.5 millivolts per degree Centigrade.

Sum 13.71 volts 3 millivolts per degree Centigrade.

Thus, this example gives negative coefficient but of rather lowermagnitude.

As a, further example, consider a case with R infinity (i.e., opencircuit) so that we revert effectively to FIG. 6, but now with differentresistance values, say R 2,500 ohms and R, 300 ohms, so that as comparedwith FIG. 6 as analyzed above, R is high as compared As before,regulation cannot commence until 0.7 volts exists across R for R 300ohms, 2% milliamps is required through R As before, the current in Rvaries with temperature because the base-emitter voltage varies withtemperature. More strictly, the base-emitter voltage is 0.7 volts 2%millivolts per degree Centigrade. Therefore, the current in R is givenby Ohms law as 1m 0.7 volts '12: millivolts poi ilvg'i'vo (toutipiimlvThe voltages across the Zener diode and baseemitter will be as before,i.e., a total of 8 volts 0.7 volts plus 5.6 millivolts per degreeCentigrade (for Zener) minus 2.5 millivolts per degree Centigrade (forVEB) V 5.83 volts 20.8 millivolts per degree Centigrade V 8.0 volts 5.6millivolts per degree Centigrade V 0.7 volts 2.5 millivolts per degreeCentigrade Total volts 14.53 17.7 millivolts per degree Centigrade.

The complete circuit thus has a negative temperature coefficient ofminus 17.7 millivolts per degree Centigrade.

As should now be apparent, the objects initially set forth at the outsetof the specification have been successfully achieved in that Applicantshave clearly shown how a selection of resistance values in a circuitotherwise similar to the prior art, achieves a new, unobvious, anddesirable result in that a regulatory circuit having a net negativetemperature coefficient or at' least a zero temperature coefficient isachieved without the use of thermistor elements. Accordingly Having thusdescribed our invention what we claim as new and desire to secure byLetters Patent is:

l. A voltage regulator for use in a battery charging system on-a roadvehicle, comprising in combination first and second supply lines forconnection to the vehi cle battery, a first resistor, a Zener diode anda second resistor connected in series between said supply lines, saidZener diode having a positive temperature coefficient, a third resistorconnected across the series combination of the Zener diode and secondresistor, an input transistor having its base-emitter circuit connectedacross said second resistor, the base-emitter circuit of said inputtransistor having a negative temperature coefficient substantially lessthan the positive temperature coefficient of said Zener diode, controlmeans coupled to the collector of said input transistor for controllingcharging of' said battery in accordance with the conduction of saidinput transistor, and said negative temperature coefficient of saidbase-emitter circuit'of the input transistor being multiplied by afactor dependent on the ratio of the resistance values of said firstresistor and said second resistor, whereby said negative temperaturecoefficient is at least equal to the positive temperature coefficient ofthe Zener diode.

2. A battery charging system for a road vehicle, comprising incombination a battery, a generator incorporating a field winding, meanscoupling said generator to said battery whereby said generator chargessaid battery, a first resistor, a Zener diode and a second resistorconnected in series across said battery, a third resistorconnectedacross the series combination of said Zener diode and said secondresistor, an input transistor having a base, a collector and an emitter,means connecting said base and emitter across said second resistor, anoutput transistor having said field winding in its collector circuit,means coupling the input transistor to the output transistor wherebyconduction of the input transistor controls conduction of the outputtransistor, a positive feedback circuit between the output and inputtransistors whereby the circuit oscillates between one state with theoutput transistor fully on and the input transistor fully off, andanother state with the output transistor off and the input transistorfully on, the

' coefficient which is substantially greater than the negativetemperature coefficient of the base-emitter of said input transistor,and said first resistor having a substantially larger resistance thansaid second resistor, whereby the effective negative temperaturecoefficient of said base-emitter is increased by a factor determined bythe ratio of the first and second resistors to a value at least equal tothe positive temperature coefficient of said Zener diode.

3. A battery charging system for a road vehicle, comprising incombination a battery, a generator incorporating a field winding, meanscoupling said generator to said battery whereby said generator chargessaid battery, a first resistor, a Zener diode and a second resistorconnected in series across said battery, a third resistor connectedacross the series combination of said Zener diode and said secondresistor, an input transistor having a base, a collector and an emitter,means connecting said base and emitter across said second resistor, an

output transistor having said field winding in its collector circuit,means coupling the input transistor to the output transistor wherebyconduction of the input transistor controls conduction of the outputtransistor,

a positive feedback circuit between the output and input transistorswhereby the circuit oscillates between one state with the outputtransistor fully on and the input transistor fully off, and anotherstate with the output transistor off and the input transistor fully on,the periods of conduction of the input and output input transistor, andsaid first resistor having a substantially larger resistance than saidsecond resistor, whereby the effective negative temperature coefficientof said base-emitter is increased by a factor determined by the ratio ofthe first and second resistors to a value at least equal to thepositivetemperature coefficient of said Zener diode, the system furtherincluding a fourth resistor and a'second Zener diode connected in seriesacross the series combination of said first resistor and first Zenerdiode, the ratio of said first resistor to said second resistor beingchosen so that until a predetermined temperature is reached the overallcircuit has a zero temperature coefficient, but said second Zener diodeconducting at said predetermined temperature and the ratio of saidfourth resistor to said second resistor being chosen so that above saidpredetermined temperature the overall circuit has a negative temperaturecoefficient.

4. A battery charging system for use in a road vehicle, comprising incombination first and second supply lines, a battery having its positiveterminal connected to the first supply line and its negative terminalconnected to the second supply line, an alternator, a full waverectifier connected to the alternator and supply-' ing ow er to thefirst and second sulppl lines, a third sup ly lme, means connecting saidt l supply line to said first supply line through a warning lamp and anignition switch of the vehicle in series, a plurality of diodesconnected to the phase points of said alternator and supplying power tosaid third supply line, whereby when the alternator is operating thepotential of said third supply line is equal to the potential of saidfirst supply line so that said warning lamp is extinguished, a seriescircuit connected between said first and second supply lines andincluding a first resistor, a Zener diode and a second resistor, a thirdresistor connected across the series combination of said Zener diode andsaid second resistor, an input transistor having its base connected tothe junction of the Zener diode and second resistor and its emitterconnected to the second line, a fourth resistor coupling the collectorof said input transistor to said third supply line, a second transistorhaving its base connected to the collector of the input transistor, itscollector connected through a fifth resistor to the third supply lineand its emitter connected to the base of an output transistor, saidoutput transistor having its emitter connected to the second supply lineand its collector connected to the third supply line through a fieldwinding of said alternator, a diode bridging said field winding, saiddiode conducting energy stored in said field winding when said outputtransistor is off, and a positive feedback circuit coupling thecollector of said output transistor to the base of said inputtransistor, the circuit incorporating the input transistor, secondtransistor and output transistor oscillating by virtue of said positivefeedback circuit to establish a mean current flow in said field winding,said mean current flow depending on the current flow through said Zenerdiode, which depends on the voltage between the first and second supplylines, the base-emitter of said input transistor having a negativetemperature coefficient, and said Zener diode having a positivetemperature coefficient, and said first resistor being substantiallygreater in magnitude than said second resistor whereby the negativetemperature coefficient of said base-emitter is multiplied by a factordetermined by the ratio of the values of the first and second resistors,to a value at least equal to the positive temperature coefficient ofsaid Zener diode.

1. A voltage regulator for use in a battery charging system on a roadvehicle, comprising in combination first and second supply lines forconnection to the vehicle battery, a first resistor, a Zener diode and asecond resistor connected in series between said supply lines, saidZener diode having a positive temperature coefficient, a third resistorconnected across the series combination of the Zener diode and secondresistor, an input transistor having its base-emitter circuit connectedacross said second resistor, the base-emitter circuit of said inputtransistor having a negative temperature coefficient substantially lessthan the positive temperature coefficient of said Zener diode, controlmeans coupled to the collector of said input transistor for controllingcharging of said battery in accordance with the conduction of said inputtransistor, and said negative temperature coefficient of saidbase-emitter circuit of the input transistor being multiplied by afactor dependent on the ratio of the resistance values of said firstresistor and said second resistor, whereby said negative temperaturecoefficient is at least equal to the positive temperature coefficient ofthe Zener diode.
 2. A battery charging system for a road vehicle,comprising in combination a battery, a generator incorporating a fieldwinding, means coupling said generator to said battery whereby saidgenerator charges said battery, a first resistor, a Zener diode and asecond resistor connected in series across said battery, a thirdresistor connected across the series combination of said Zener diode andsaid second resistor, an input transistor having a base, a collector andan emitter, means connecting said base and emitter across said secondresistor, an output transistor having said field winding in itscollector circuit, means coupling the input transistor to the outputtransistor whereby conduction of the input transistor controlsconduction of the output transistor, a positive feedback circuit betweenthe output and input transistors whereby the circuit oscillates betweenone state with the output transistor fully on and the input transistorfully off, and another state with the output transistor off and theinput transistor fully on, the periods of conduction of the input andoutput transistors being determined by the current flow through saidZener diode, the base-emitter of said input transistor having a negativetemperature coefficient and said Zener diode having a positivetemperature coefficient which is substantially greater than the negativetemperature coefficient of the base-emitter of said input transistor,and said first resistor having a substantially larger resistance thansaid second resistor, whereby the effective negative temperaturecoefficient of said base-emitter is increased by a factor determined bythe ratio of the first and second resistors to a value at least equal tothe positive temperature coefficient of said Zener diode.
 3. A batterycharging system for a road vehicle, comprising in combination a battery,a generator incorporating a field winding, means coupling said generatorto said battery whereby said generator charges said battery, a firstresistor, a Zener diode and a second resistor connected in series acrosssaid battery, a third resistor connected across the series combinationof said Zener diode and said second resistor, an input transistor havinga base, a collector and an emitter, means connecting said base andemitter across said second resistor, an output transistor having saidfield winding in its collector circuit, means coupling the inputtransistor to the output transistor whereby conduction of the inputtransistor controls conduction of the output transistor, a positivefeedback circuit between the output and input transistors whereby thecircuit oscillates between one state with the output transistor fully onand the input transistor fully off, and another state with the outputtransistor off and the input transistor fully on, the periods ofconduction of the input and output transistors being determined by thecurrent flow through said Zener diode, the base-emitter of said inputtransistor having a negative temperature coefficient and said Zenerdiode having a positive temperature coefficient which is substantiallygreater than the negative temperature coefficient of the base-emitter ofsaid input transistor, and said first resistor having a substantiallylarger resistance than said second resistor, whereby the effectivenegative temperature coefficient of said base-emitter is increased by afactor determined by the ratio of the first and second resistors to avalue at least equal to the positive temperature coefficient of saidZener diode, the system further including a fourth resistor and a secondZener diode connected in series across the series combination of saidfirst resistor and first Zener diode, the ratio of said first resistorto said second resistor being chosen so that until a predeterminedtemperature is reached the overall circuit has a zero temperaturecoefficient, but said second Zener diode conducting at saidpredetermined temperature and the ratio of said fourth resistor to saidsecond resistor being chosen so that above said predeterminedtemperature the overall circuit has a negative temperature coefficient.4. A battery charging system for use in a road vehicle, comprising incombination first and second supply lines, a battery having its positiveterminal connected to the first supply line and its negative terminalconnected to the second supply line, an alternator, a full waverectifier connected to the alternator and supplying power to the firstand second supplY lines, a third supply line, means connecting saidthird supply line to said first supply line through a warning lamp andan ignition switch of the vehicle in series, a plurality of diodesconnected to the phase points of said alternator and supplying power tosaid third supply line, whereby when the alternator is operating thepotential of said third supply line is equal to the potential of saidfirst supply line so that said warning lamp is extinguished, a seriescircuit connected between said first and second supply lines andincluding a first resistor, a Zener diode and a second resistor, a thirdresistor connected across the series combination of said Zener diode andsaid second resistor, an input transistor having its base connected tothe junction of the Zener diode and second resistor and its emitterconnected to the second line, a fourth resistor coupling the collectorof said input transistor to said third supply line, a second transistorhaving its base connected to the collector of the input transistor, itscollector connected through a fifth resistor to the third supply lineand its emitter connected to the base of an output transistor, saidoutput transistor having its emitter connected to the second supply lineand its collector connected to the third supply line through a fieldwinding of said alternator, a diode bridging said field winding, saiddiode conducting energy stored in said field winding when said outputtransistor is off, and a positive feedback circuit coupling thecollector of said output transistor to the base of said inputtransistor, the circuit incorporating the input transistor, secondtransistor and output transistor oscillating by virtue of said positivefeedback circuit to establish a mean current flow in said field winding,said mean current flow depending on the current flow through said Zenerdiode, which depends on the voltage between the first and second supplylines, the base-emitter of said input transistor having a negativetemperature coefficient, and said Zener diode having a positivetemperature coefficient, and said first resistor being substantiallygreater in magnitude than said second resistor whereby the negativetemperature coefficient of said base-emitter is multiplied by a factordetermined by the ratio of the values of the first and second resistors,to a value at least equal to the positive temperature coefficient ofsaid Zener diode.